In U.S. Pat. No. 4,251,130 and in the copending application and by P.P. Liu et al, Ser. No. 161, 761, filed June 23, 1980, assigned to applicant's assignee, coupled circuits of the velocity-matched type (VMG) are disclosed. In the Marcatili application, coupling efficiency is controlled by a traveling electrical wave that propagates in synchronism with an optical signal. In the Liu et al application, the electrical circuit is tuned so as to produce a standing electrical wave. However, the two oppositely propagating waves that produce the standing wave are also velocity matched to the optical signal. Because these devices are capable of generating very narrow optical pulses, they have been characterized as "gates." More specifically, the Marcatili device is referred to as a "traveling-wave, velocity-matched gate" (TW-VMG) while the Liu et al device is called a "standing-wave, velocity-matched gate" (SW-VMG).
The general problem with both classes of device resides in the fact that the refractive indices of the substrate material in which the circuit is formed may be very different at the electrical signal and optical signal frequencies. In such cases, the electrical wavepath must be specially designed to satisfy the velocity match requirement and, to the extent that the velocities of the two signals are not matched, the coupling efficiency degenerates significantly.
An alternative approach to this problem is to simulate a velocity match while, in fact, the two signals propagate at very different velocities. One arrangement for accomplishing this, as disclosed in a second copending application by E. A. J. Marcatili, Ser. No. 174,831, filed Aug. 4, 1980, and assigned to applicant's assignee, is to form a standing wave pattern along the electrical signal wavepath and then to shape the wavepath so that interaction between the standing electrical wave and the propagating optical wave occurs only along selected regions of the electrical signal wavepath. The net effect is to produce a standing-wave, velocity-mismatched gate (SW-VMMG) whose mode of operation is equivalent to that of a standing-wave, velocity-matched gate.
The TW-VMG is broadband in both the electrical and optical circuits. As such, it can be used as a broadband modulator, as described in U.S. Pat. No. 4,005,927, or it can be employed to generate ultranarrow optical pulses, as taught by Marcatili.
The SW-VMG and the SW-VMMG while being broadband in the optical circuit are narrowband in the electrical circuit. Because both employ standing waves, the electrical circuits are tuned circuits. In addition, the VMMG electrode configuration is designed for a specific wavelength electrical signal. Thus, the SW-VMG and the SW-VMMG are particularly suitable for generating narrow optical pulses by means of a single frequency electrical signal.
While the traveling-wave, velocity-matched configuration can be used as a broadband modulator, it requires, as noted above, that the electrical circuit be modified so as to achieve the necessary match. Using a lithium niobate substrate, the effective refractive index at optical frequencies is approximately 2.2 whereas it is approximately 6 at microwave frequencies. Thus, some means must be provided for lowering the effective index at microwave frequencies. One technique is to load the electrical circuit with a low index dielectric. However, the effect is to concentrate the electrical field in the lower index material rather than in the LiNiO.sub.3, where it is required. As a result, such circuits are wasteful of microwave power. What is desired in a broadband modulator is both a broadband electrical circuit combined with efficient use of the electrical power.